1. Field of the Invention
The present invention relates generally to the energy field, and more specifically, to a processor for drying and heating coal and mixing it with cool (non-dried) coal.
2. Description of the Related Art
Coal is increasingly in demand as an immediately available source of incremental energy to fuel the world's growing energy needs. Coal has and will continue to increase in price as all other sources of energy, particularly petroleum, are depleted and increase in value. Both the US domestic and global coal markets are changing as existing high-grade coal sources are depleted. As a result, utility and other industrial users of coal are spending large amounts of capital to refit existing plants or build new plants designed to burn lower quality (rank) coals, or paying increasingly higher amounts for high-grade compliance coals that better meet the optimal operational specifications.
Coal upgrading (converting a low-rank coal to a higher rank coal) provides viable access to the great resources of lower rank coals available in the United States and other countries and provides a low-cost alternative to either extensive modifications needed to handle and combust the lower rank coals, or a reduction in the productive capacity of the existing power plant facilities suffered when the lower rank coals are used without alteration.
Under the right conditions of temperature and pressure, organic matter in nature undergoes a metamorphous, or coalification, process as peat is gradually converted to lignite, sub-bituminous coal, bituminous coal, and finally to anthracite. This transition—in which the rank of the coal increases—is characterized by a decrease in the moisture and oxygen content of the coal and an increase in the carbon-to-hydrogen ratio. Lignite and sub-bituminous coals have not been as thoroughly metamorphosed and typically have high inherent (bound) moisture and oxygen contents and, correspondingly, produce less combustive heat energy per ton of coal.
All coals were deposited in marine environments where non-combustible impurities such as clay, sand, and other minerals are interbedded with the organic material and form ash in the combustion process, contributing to deposit formation on the system heat exchange surfaces. Additionally, some combustible materials such as pyrite are deposited within the coal by a secondary geologic process. It is these impurities that are responsible for the production of much of the sulfur dioxide, particulates and other pollutants when burning coals. These impurities exist in all ranks of coals, requiring expensive pollution controls technologies to be employed to reduce the level of emissions in the released flue gas to be compliant with the regulatory mandates.
The combustion system designed for a particular coal will not work as effectively for a coal of dissimilar rank or quality. For a specific heat release rate, the furnace volume required for combustion decreases with increasing rank. Because each combustion system performs well when consuming a coal with specific rank and quality (ash content) characteristics, firing with a coal that does not conform to the design fuel typically results in reducing the efficiency of the system. As the concentration of the mineral impurities (or ash content) increases, the operational characteristics of the combustion system are detrimentally affected. Additionally, the system produces increasing quantities of hazardous pollutants that must be captured to prevent release into the environment.
Coal drying technologies raise the apparent rank of the feed coal processed by reducing the moisture content of the coal which results in more heat produced per ton of dried—or upgraded—coal. Certain processes also reduce oxygen and volatile content. This is generally accomplished using a system in which the coal is dried with an inert gas (i.e., a gas with no oxygen concentration) or a gas having an acceptably low concentration of oxygen.
Coal cleaning processes reduce the concentration of mineral impurities in the processed coal. In the ideal case, only mineral matter would be removed from the organic material, leaving only organic material. The efficiency of the cleaning process is dependent on the extent to which mineral matter is liberated (physically separated into discrete particles that are predominantly mineral matter or organic material) from the organic material. In practice, mineral particles will not be predominately liberated from the organic material, particularly in the lower rank coals. As such, it is not possible to completely separate all of the mineral matter from the organic material without losing organic material also. Cleaning is not typically applied to low-rank coals because of the relative abundance and low value of the native or unprocessed low-rank coals and because simply crushing a low-rank coal does not effectively liberate mineral matter from the organic material.
The American Society of Testing and Materials provides procedures for analyzing coal samples. Moisture content is defined as the loss in mass of a sample when heated to 104° C. Volatile content is defined as the loss in mass of a sample when heated to 950° C. in the absence of air, less the moisture content. The ash content is defined as the residue remaining after igniting a sample at 750° C. in air. As a sample is heated, moisture is evolved from the sample concurrent with an increase in the temperature of the coal remaining. If the sample is allowed to maintain an equilibrium between the temperature of the coal and the moisture content, all of the moisture would be removed when the coal residue has a temperature of 104° C. As the coal is heated further in the absence of oxygen, volatile organic compounds (VOCs), a regulated hazardous air pollutant) are evolved.
Numerous schemes have been devised to upgrade—or dry—low-rank coals. These attempts can be divided into three levels of effort: partial drying, complete drying, and complete drying with additional volatile content removed. As noted above, the processing temperature of the final dried product will typically increase in relation to the extent of processing; that is, the final product temperature of a partially dried coal will be lower than would be expected for the final product temperature of the same coal dried completely. The temperature of the process gas used by many processes has historically been elevated to minimize the contact time between the coal and the process gas required to dry the coal; however, this in turn causes VOCs to be stripped from the coal particles as the outside portion of the particles will tend to be heated to a higher temperature than the inside of the particles. A high-temperature process gas may not be used in driers with relatively short drying times if the elimination of VOCs is a desired result.
Numerous methods have been devised to heat the coal: direct contact with a relatively inert gas, indirect contact with a heated fluid medium, hot oil baths; etc. Some processes operate under vacuum while some operate at elevated pressure. Regardless of the process, the dried product qualities are relatively similar, and the costs are prohibitive. To be economically attractive, the total processing cost, including the costs of the feed coal and the environmental controls, cannot exceed the cost of an available higher rank coal delivered to the customer.
The dried product resulting from the majority, if not all, of the conventional processes have four attributes that reduce the value of the dried product. The dried product is typically dusty, prone to moisture re-absorption, prone to spontaneous ignition, and has a reduced bulk density. These characteristics require special attention relating to handling, shipping and storage.
With few exceptions, notably indirectly heated screw augers and rotary kiln drying, many of the conventional processes require a sized feed with the largest particle size or the smallest particle size limited to accommodate processing constraints. Fluidized bed and vibrating fluidized bed processes, while efficient for contacting the drying media with the coal, do not tolerate fines due to elutriation. Fluidized beds do not operate efficiently when processing particles with a wide size range; oversized material requires increased compressive power, and fine material is elutriated from the fluidized bed processor.
The inability to produce a dried product at an acceptable cost has prevented these processes from gaining reasonable commercial acceptability. Capital and operating costs, together with product quality issues (e.g., the coal is dusty, prone to spontaneous ignition, etc.), have resulted in the perception that coal upgrading should not be included in the discussion relating to increasing available high-quality, low-cost fuel supplies, which may extend the life and expand the productive capacity of some combustion systems while reducing the uncontrolled emission inventory.
Further, as the extent, or intensity, of processing increases (final product temperature increases), the environmental processing costs increase because the evolution of VOCs demands pollution control systems, and the materials of construction require additional capital to accommodate the elevated temperatures and corrosive environment.
Disregarding the cost of feed coal and the cost of heat energy, operating costs for coal upgrading have historically been quite high. High compressive energy costs are typically associated with fluid and vibrating fluid beds. High maintenance costs are typically associated with higher temperatures and more corrosive environments. High labor costs are usually a function of maintenance requirements and complicated process configurations. All of these issues combine to increase process controls and supervision costs.
The dried product from the conventional processes varies in the qualities desired for a cleaning process. A coarser product is more amenable to the cleaning system because separation is a function of particle size, shape and density. This requires the coal to be sized for delivery to the cleaning system and precludes cleaning the very small sizes. Fluid bed product is not a particularly good feed for cleaning systems because a large portion of the product particles are too small to be cleaned efficiently.
Product cooling has not been given the level of consideration warranted by dried coal properties. Regulations for coal transported in marine vessels requires the coal not exceed 140° F. to avoid fires on the vessel. Cooling the dried product represents a significant cost, and many of the unit operations attempted have not been particularly effective for reducing the temperature of the dried product to acceptable temperatures for transporting, handling and storing the dried product.
Producing a dried coal that has consistent qualities throughout the size range of the particles with five percent (5%) of the moisture content that was present in the parent or feed coal while limiting the evolution of VOCs to negligible levels would be highly desirable. This would limit the environmental processing to particulate considerations. Processing the feed coal by direct contact with a relatively inert gas at a temperature of about 700° F. would allow flue gas from industrial or utility systems to be used while minimizing costs related to materials of construction and reducing process gas volumes to be handled.